The present invention relates to fuel cells that are suited for usage in transportation vehicles, portable power plants, or as stationary power. plants, and the invention especially relates to a fuel cell that utilizes an antifreeze solution passing through the fuel cell to remove heat from the cell.
Fuel cell power plants are well-known and are commonly used to produce electrical energy from reducing and oxidizing fluids to power electrical apparatus such as apparatus on-board space vehicles. In such power plants, a plurality of planar fuel cells are typically arranged in a stack surrounded by an electrically insulating frame structure that defines manifolds for directing flow of reducing, oxidant, coolant and product fluids. Each individual cell generally includes an anode electrode and a cathode electrode separated by an electrolyte. A reactant or reducing fluid such as hydrogen is supplied to the anode electrode, and an oxidant such as oxygen or air is supplied to the cathode electrode. In a cell utilizing a proton exchange membrane (xe2x80x9cPEMxe2x80x9d) as the electrolyte, the hydrogen electrochemically reacts at a surface of the anode electrode to produce hydrogen ions and electrons. The electrons are conducted to an external load circuit and then returned to the cathode electrode, while the hydrogen ions transfer through the electrolyte to the cathode electrode, where they react with the oxidant and electrons to produce water and release thermal energy.
The anode and cathode electrodes of such fuel cells are separated by different types of electrolytes depending on operating requirements and limitations of the working environment of the fuel cell. One such electrolyte is the aforesaid proton exchange membrane (xe2x80x9cPEMxe2x80x9d) electrolyte, which consists of a solid polymer well-known in the art. Other common electrolytes used in fuel cells include phosphoric acid or potassium hydroxide held within a porous, non-conductive matrix between the anode and cathode electrodes. It has been found that PEM cells have substantial advantages over cells with liquid acid or alkaline electrolytes in satisfying specific operating parameters because the membrane of the PEM provides a barrier between the reducing fluid and oxidant that is more tolerant to pressure differentials than a liquid electrolyte held by capillary forces within a porous matrix. Additionally, the PEM electrolyte is fixed, and cannot be leached from the cell, and the membrane has a relatively stable capacity for water retention.
Manufacture of fuel cells utilizing PEM electrolytes typically involves securing an appropriate first catalyst layer, such as a platinum alloy, between a first surface of the PEM and a first or anode porous substrate or support layer to form an anode electrode adjacent the first surface of the PEM, and securing a second catalyst layer between a second surface of the PEM opposed to the first surface and a second or cathode porous substrate or support layer to form a cathode electrode on the opposed second surface of the PEM. The anode catalyst, PEM, and cathode catalyst secured in such a manner are well-known in the art, and are frequently referred to as a xe2x80x9cmembrane electrode assemblyxe2x80x9d, or xe2x80x9cM.E.A.xe2x80x9d, and will be referred to herein as a membrane electrode assembly. In operation of PEM fuel cells, the membrane is saturated with water, and the anode electrode adjacent the membrane must remain wet. As hydrogen ions produced at the anode electrode transfer through the electrolyte, they drag water molecules in the form of hydronium ions with them from the anode to the cathode electrode or catalyst. Water also transfers back to the anode from the cathode by osmosis. Product water formed at the cathode electrode is removed from the cell by evaporation or entrainment into a gaseous stream of either the process oxidant or reducing fluid.
While having important advantages, PEM cells are also known to have significant limitations especially related to liquid water transport to, through and away from the PEM, and related to simultaneous transport of gaseous reducing fluids and process oxidant fluids to and from the electrodes adjacent opposed surfaces of the PEM. The prior art includes many efforts to minimize the effect of those limitations. Use of such fuel cells to power a transportation vehicle gives rise to additional problems associated with water management, such as preventing the product water from freezing, and rapidly melting any frozen water during start up whenever the fuel-cell powered vehicle is operated in sub-freezing conditions.
Known fuel cells typically utilize a coolant system supplying a flow of cooling fluid through a cooler plate within the fuel cell to maintain the cell within an optimal temperature range. The cooler plate may be sealed as described in U.S. Pat. No. 5,804,326 that issued on Sep. 8, 1988 to Chow, et al., or the plate may be in diffusable communication with the electrodes or other fuel cell components as described in U.S. Pat. No. 5,503,944 that issued on Apr. 2, 1996 to Meyer et al., which patent is owned by the assignee of all rights in the fuel cell with a direct antifreeze impermeable cooler plate invention described herein. Where the cooling fluid is a solution including water it also must be kept from freezing. It is known to utilize a conventional antifreeze solution such as ethylene glycol and water or propylene glycol and water as a cooling fluid in sealed coolant systems utilizing sealed cooler plates. However, such antifreeze solutions are not viable in cells with diffusable contact between the cooling fluid and fuel cell components because those antifreeze solutions are known to be adsorbed by and poison the catalysts that form electrodes in the cell. Furthermore, those antifreeze solutions have low surface tensions which result in the solutions wetting any porous, wetproofed support layers adjacent cell catalysts, thereby impeding diffusion of reactant fluids through the support layers to the catalysts, which further decreases performance of the electrodes. Also, the vapor pressure of such conventional antifreezes is high, resulting in excessive loss rates of the antifreeze solutions through fuel cell exhaust streams.
It is known and desirable to use commercial graphite, or graphite-polymer composites to form cooler plates in order to both direct and contain a flow of conventional organic antifreezes through a fuel cell. However, under fuel cell operating conditions that include an electrochemical potential, such graphite materials often develop a through-plane porosity which permits conventional antifreeze solutions to wick due to capillary action or due to a positive pressure differential from the cooler plate into adjacent cell components and to cell catalysts, thereby poisoning the catalysts and/or impeding movement of reactant and product fluids through the components, resulting in a decrease in overall cell performance. Consequently, coolant systems of fuel cells that utilize a conventional antifreeze solution are known to be sealed from the electrodes, so that the solution is not in direct fluid communication with the electrodes.
Sealed cooler plates are commonly known to be metal in order to prevent loss of the conventional antifreeze from the plate into adjacent cell components. In PEM fuel cells, it is known that the water within the cell frequently has a pH of between 4.0-4.5. Such a pH along with ordinary cell potentials results in common heat exchanger metals such as copper, iron, steel or aluminum being unacceptable due to excessive corrosion rates. Stainless steels have suitable corrosion resistance, however, it is known that their surfaces form a nonconductive passivation layer that increases resistance of cooler plates fabricated of stainless steels to unacceptable levels. Therefore, expensive platings of noble metals such as gold are frequently used that are known to be both costly and unreliable. It is also known to use transition metals such as tantalum, titanium, niobium, etc. or alloys thereof as a base plated with gold or platinum, or to use gold or titanium alloys alone, which is also quite costly.
In fuel cell stacks utilizing a plurality of such metal cooler plates that are impermeable to conventional antifreeze solutions, the manifolds that distribute the antifreeze between various cooler plates within the cell stack must include seals against loss of the antifreeze into fuel cell components that would result in degradation of fuel cell performance. In a common fuel cell stack assembly containing typically 200-500 cells, there will be several hundred antifreeze coolant seals within the cell stack assembly. Because of high vapor pressure and low surface tension of conventional antifreezes, known antifreeze coolant seals must therefore be complicated and hence expensive.
Accordingly there is a need for a fuel cell that may be operated in sub-freezing conditions that does not require a costly, unreliable antifreeze impermeable cooler plate.
A fuel cell with a direct antifreeze impermeable cooler plate is disclosed for producing electrical energy from reducing fluid and process oxidant reactant streams. The fuel cell includes an electrolyte secured between an anode catalyst and a cathode catalyst; an anode flow field secured adjacent the anode catalyst for directing the reducing fluid to pass adjacent the anode catalyst; a cathode flow field secured adjacent the cathode catalyst for directing the process oxidant stream to pass adjacent the cathode catalyst; a direct antifreeze impermeable cooler plate secured in heat exchange relationship with the cathode flow field; and a direct antifreeze solution passing through the cooler plate for controlling temperature within the fuel cell, wherein the direct antifreeze solution is an organic antifreeze solution that is non-volatile at cell operating temperatures. For purposes herein, xe2x80x9cnon-volatilexe2x80x9d is defined to mean that the antifreeze solution sustains a loss of less than 10% of its antifreeze for every 500 operating hours of the fuel cell at fuel cell operating temperatures.
The direct antifreeze impermeable cooler plate may be any material that is impermeable to liquid and compatible with a fuel cell operating environment such as plated metals, or in a preferred embodiment, the cooler plate may be a fine pore graphite material having a mean pore size of less than one micron and a porosity of less than twenty-five per cent. In a further preferred embodiment, the direct antifreeze solution may be a special direct antifreeze solution having the following characteristics: 1. a freezing point of at least xe2x88x9220 degrees Fahrenheit (hereafter xe2x80x9cxc2x0 F.xe2x80x9d); 2. a surface tension greater than 60 dynes per centimeter (hereafter xe2x80x9cdyne/cmxe2x80x9d) at a cell operating temperature of about 150xc2x0 F.; 3. a partial pressure of antifreeze above the solution at about 150xc2x0 F. that is less than 0.005 mm of mercury (hereafter xe2x80x9cmm Hgxe2x80x9d); and, 4. that is capable of being oxidized by catalysts of the fuel cell at fuel cell voltages. A second preferred antifreeze solution may be an alkanetriol direct antifreeze solution, and in particular an alkanetriol selected from the group consisting of glycerol, butanetriol, and pentanetriol. The direct, special direct, and alkanetriol direct antifreeze solutions have high surface tension and low volatility characteristics that facilitate usage of uncomplicated seals in manifolds that deliver the direct antifreeze solutions to the fuel cell cooler plate. In the event the cooler plate is made of a fine pore graphite material, the favorable high surface tension and low volatility characteristics of the direct antifreeze solution also restrict movement of the antifreeze as a liquid through the cooler plate into other fuel cell components.
In an additional embodiment, the direct antifreeze solution passing through the cooler plate may be directed to flow at a pressure that is less than a pressure of the process reactant streams passing through the fuel cell. A preferred fuel cell operates for example at near ambient pressure and the process oxidant stream and reducing fluid stream are pressurized to 1 to 2 pounds per square inch gauge (hereafter xe2x80x9cPSIGxe2x80x9d) above ambient pressure, and the direct antifreeze solution is directed to flow through the direct antifreeze impermeable cooler plate at about 1 to 2 PSIG below ambient pressure. Such a positive pressure differential between the reactant streams and the direct antifreeze solution within the cooler plate further restricts movement of the direct antifreeze solution out of the cooler plate.
Accordingly it is a general object of the present invention to provide a fuel cell with a direct antifreeze impermeable cooler plate that overcomes deficiencies of the prior art.
It is a more specific object to provide a fuel cell with a direct antifreeze impermeable cooler plate that permits usage of a graphite cooler plate.
It is yet another object to provide a fuel cell with a direct antifreeze impermeable cooler plate that permits usage of uncomplicated seals in manifolds passing the direct antifreeze through the fuel cell.
It is another object to provide a fuel cell with a direct antifreeze impermeable cooler plate that restricts movement of the direct antifreeze out of the cooler plate into fuel cell components.
These and other objects and advantages of the present fuel cell with a direct antifreeze impermeable cooler plate will become more readily apparent when the following description is read in conjunction with the accompanying drawings.